Refrigeration process are commonly used in industries to: liquefy gases like oxygen, nitrogen, propane and methane, purify compressed air of moisture, maintain a low process temperature in oil/petrochemical/chemical processes, and for tempering metal in metallurgy industry. The chiller apparatus commonly used to obtain refrigeration are broadly based on vapor compression or vapor absorption principle. These apparatus utilize the ability of liquids or salts to absorb vapors of a working fluid and give heating and/or cooling effect.
The vapor absorption chiller apparatus are thermally driven, which means that heat from waste source or heat derived from solar collectors rather than mechanical energy is used to drive the cycle; whereas, the vapor compression chiller apparatus use high grade energy (electrical energy) from mechanical inputs. Thus, the basic difference between an absorption chiller apparatus and a compression chiller apparatus is that the compression chiller uses an electric motor for operating a compressor for raising the pressure of refrigerant vapors whereas an absorption chiller uses heat for compressing the refrigerant vapors to a high pressure. Thus, absorption chiller apparatus are more economical and environmental friendly since they employ low-grade waste heat, conserve electricity, and use non-ozone depleting refrigerants (water); however, compression chiller apparatus are more preferred due to their higher coefficient of performance (COP).
The need for energy conservation has been highlighted by concerns about the environment, leading development towards energy efficient refrigeration systems. Increased attention has been directed towards development of cost-effective and efficient systems for providing refrigeration, thus, reducing the consumption of energy/power. As a result, the absorption chiller apparatus are gaining favor over conventional compression chiller apparatus in industrial applications, as they use little energy and are environmental friendly. However, in a refrigeration system, when sub-zero evaporation temperatures are desired, a compression chiller apparatus is suitable, but, the power consumptions will be high, while, an absorption chiller apparatus cannot provide sub-zero evaporation temperatures. Also, a lithium-bromide/water based absorption chiller cannot be used where the circulated cooling water temperature is above 40[deg.] C.
In recent developments aimed at overcoming the above listed problems, absorption chillers have been combined with compression chillers to provide cooling at lowest energy costs. Such hybrid chiller, thus devised, operates the absorption chiller during high electric peak load when the charges are high, whereas the compression chiller is operated during low electric peak load when the charges are low, thereby providing a more economical system. A basic refrigeration cycle for a hybrid absorption-compression chiller uses a low temperature liquid refrigerant that absorbs heat from water, air, or any medium to be cooled, and converts to a vapor phase in an evaporator section. The refrigerant vapors are then compressed to a high pressure, by a compressor or a generator, converted back to a liquid by rejecting heat to the external surrounding, in a condenser section, and then expanded to a low-pressure mixture of liquid and vapor, that goes back to the evaporator section and the cycle is repeated.
Several attempts have been made in the past to provide a hybrid absorption-compression chiller which provides refrigeration effect. Some of the disclosures are listed in the prior art below:
U.S. Pat. No. 7,765,823 discloses a hybrid vapor compression-absorption cooling or heating system and apparatus thereof, employing a refrigerant pair comprising at least one refrigerant and at least one ionic liquid. The apparatus comprises: an absorber that forms a mixture of a refrigerant and an absorbent, a generator that heats the mixture and separates the refrigerant vapors, a condenser that receives the vapors and condenses them to a liquid, a pressure reduction device to reduce the pressure of the liquid refrigerant to form a mixture of liquid and vapor refrigerant, an evaporator that receives the mixture to evaporate the remaining liquid and provide a first and a second portion of refrigerant vapors, a compressor that receives the first portion of the vapors increases the pressure and passes to the condenser, a conduit that passes the second portion of the vapors to the absorber which comprises one or more ionic liquids.
U.S. Pat. No. 7,624,588 discloses a hybrid absorption-compression chiller which uses high temperature steam and medium temperature water generated in industrial processes as a heat source. The chiller comprises: a high temperature generator that uses the high temperature steam so as to exchange heat with refrigerant; a low temperature generator that uses the heat of condensation of the refrigerant steam, generated in the high temperature generator, so as to exchange heat with the refrigerant; a generator that uses medium temperature water to exchange heat with refrigerant; a hybrid condenser for condensing the refrigerants; an evaporator that uses the latent heat of vaporization of the condensed refrigerant to obtain chilled water; and an absorber that receives a strong solution and absorbs the refrigerant steam to produce a weak solution.
U.S. Pat. No. 3,824,804 discloses a refrigerating machine which is a combination of a compression type refrigerating apparatus and an absorption type refrigerating apparatus. The compression type refrigerating apparatus comprises a compressor, a condenser, a throttle valve, and an evaporator connected in series to form a first closed loop. The absorption type refrigerating apparatus comprises a generator, a condenser, an evaporator, and an absorber connected in series so as to form a second closed loop. The generator and the evaporator of the absorption apparatus are arranged as a heat exchanger. By combining the generator and the compressor into a unit a decrease in temperature can be obtained in the outlet valve of the compressor.
The hybrid absorption-compression chillers disclosed above are complex and/or are not operable at sub-zero evaporation temperatures. Therefore, there is felt a need for a hybrid absorption-compression chiller which while having a simple construction, providing higher COP and giving energy savings, is also adaptable over a wide range of operating conditions and subzero evaporation temperatures.